A duplex communication system includes two transceivers that communicate with each other over a channel in both directions. There are two types of duplex communication systems: half-duplex communication systems and full-duplex communication systems. In half-duplex communication systems, the two transceivers communicate with each other over the channel in both directions but only in one direction at a time; that is, only one of the two transceivers transmits at any given point in time, while the other receives. A full-duplex communication system, on the other hand, does not have such a limitation. Rather, in a full-duplex communication system, the two transceivers can communicate with each other over the channel simultaneously in both directions.
Wireless communication systems often emulate full-duplex communication. For example, in some wireless communication systems two transceivers communicate with each other simultaneously in both directions using two different carrier frequencies or channels. This scheme, where communication is carried out simultaneously in both directions using two different carrier frequencies, is referred to as frequency division duplexing (FDD). FDD is said to only emulate full-duplex communication because FDD uses two half-duplex channels rather than a single channel to accomplish simultaneous communication in both directions.
Although emulated full-duplex communication using FDD allows for simultaneous communication in both directions, it requires two channels. True full-duplex communication eliminates the need for one of these two channels, resulting in increased spectrum efficiency. The difficulty with true full-duplex communication, and the reason why it has not become common place in wireless and mobile communication standards to date, is the significant interference that the receiver of a full-duplex communication device will generally experience from the full-duplex communication device's own transmitter transmitting over the same channel that the receiver is to receive signals. This interference is referred to as self-interference because the interference experienced by the receiver originates from its own paired transmitter.
For example, in some communication systems, signals can be transmitted at power levels as high as 25 dBm and signals can be received at power levels as low as −100 dBm. At these levels, the self-interference needs to be reduced by at least 25 dBm−(−100 dBm)=125 dBm to allow for information to be recovered from the received signals.
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